Method for making all-plastic heat-sealable container

ABSTRACT

The invention provides a dynamic system for heat-sealing an all-plastic material in a continuous process at high speed. The invention includes an all-plastic heat-sealable container material comprising a multiple-layer composite, in sheet form, including a core layer and at least one outer layer on one side thereof. The layers preferably comprise thermoplastic material, the core layer material having a higher softening point than the outer layer material. The composite sheet is conveyed through a converting, side-seaming or other apparatus at high speed, the system being tuned to dynamically heat the outer layers to a molten state for heat sealing, while the core does not soften but remains undistorted. The core is thus thermally insensitive to heat at speeds used in side-seaming the composite, and as such it supports and maintains the structure of the sealing outer layers on its exterior while they are in a molten condition.

This is a division of application Ser. No. 702,018 filed July 2, 1976now U.S. Pat. No. 4,126,262.

This invention relates to all-plastic materials and to systems forhandling such all-plastic materials including methods for sealing orconstructing them into useful objects such as containers.

In the container field, it has been common practice to use many forms ofmaterials which are heat-sealed or glued and other wise converted onconventional equipment to form a container or carton. Such containersare those typically known in the trade as "folding boxes", containers,or cartons.

By way of example, one such carton is the gable-top milk carton andcarton blanks therefor which are specifically disclosed in U.S. Pat. No.3,120,333 as liquid-tight containers. Essentially, blanks used in themanufacture of such containers includes a paperboard base, extrusioncoated on both sides with a resin, such as polyethylene, to provide amoisture barrier and to provide means for sealing the carton.

In a typical carton converting operation, and once the resin-coatedblanks are cut and stored, the resin on an outer surface of a glue flapand the resin on an inner surface of a carton panel is heated by directflame application while the heated carton surfaces extend in guided butessentially unsupported (i.e., not compressed between two heating jaws)condition over the edges of a conveying belt. The carton panels are thenfolded over to form a flattened tube, the now molten tacky resin on theheated surfaces being pressed together at a downstream nip to form aliquid-tight side seam. The cartons, in a flattened tube form, are thenshipped to users such as dairies where they are finally erected byfurther heat-sealing, filled, and finally sealed.

While these familiar milk cartons have been extensively used over theUnited States, they exhibit several troublesome problems. Oneparticularly troublesome problem results from wetting of the paperboardcomponent of the carton. Such wetting can take place in areas where themoisture barrier resin is insufficient or fails. Pinholing of the resinand film failure or creasing along fold lines are examples of undesiredproblem areas where wetting is likely to occur. More importantly,moisture is also "wicked", or drawn by capillary action, into thepaperboard via the exposed paperboard at the edge of the glue flap whichresides within the filled carton. Wetting of the paperboardsubstantially weakens the cartons, may cause them to lose shape or toleak, and can destroy their desired appearance. While certaininnovations, such as that described in U.S. Pat. Nos. 3,482,278 and3,927,245 have substantially improved paperboard cartons, the existenceof paperboard in the carton renders the carton always subject toundesired paperboard wetting.

In one attempt to overcome this problem it has been proposed to employhomogeneous, all-plastic liquid containers such as can be formed by ablow-molding operation. Once such containers are formed, they aretypically shipped to a dairy for filling. By virtue of the fact thesecontainers are completely formed, and the fact that their transportationthus includes transporting the air in them, shipping charges aresubstantially increased over shipping charges for similar volumecontainers which can be shipped in a flattened condition. Thissubstantially limits the range over which the blow-molded containers canbe shipped from the molding plant. Moreover, not only are suchcontainers not readily adaptable to inexpensive printed decorations, ascan be appreciated, but plastic distortion and container wall weaknessesresult from the forming operations, rendering such containers even lessdesirable.

Another attempt to provide an all-plastic container is disclosed, forexample, in U.S. Pat. Nos. 3,389,849 and 3,334,802. In these patents, anall-plastic carton blank of requisite stiffness is cut and scored inpatterns similar to those of the resin-coated paperboard cartonsdisclosed in the other patents cited above. The patents disclose theforming of such all-plastic blanks into tubes by joining the side seampursuant to such broad techniques as "heat, sound or light", but they donot teach how the specific heat-sealing technique of conventionalresin-coated paperboard converting equipment can be used to side-seamsuch blanks, nor how sealing can be accomplished at high speed.

Unfortunately, the known sealing techniques broadly cited by the patentsrelating to these homogeneous plastic blanks do not provide speeds whichare necessary for commerical production, i.e., on the order of 125,000containers per hour. Moreover, when these homogeneous plastic containersare run through a typical resin-coated paperboard converter, theconventional technique of side-seaming by direct flame application tothe unconfined container surfaces does not work. In order to render theplastic surface sufficiently molten to efficiently seal, so much heatmust be applied to the plastic overall that it draws up and shrivelsinto an unusable mass.

Thus, while homogeneous all-plastic blanks could possibly be sealed bysome known heat-sealing technique such as a static system wherein theheated areas are supported or confined, for example, between heatingjaws, no such other known techniques is capable of high commercialproduction speed. Moreover, the use of other sealing techniques wouldrequire the converter to purchase other equipment to provide efficientsealing, the all-plastic container rendering his current equipmentobsolete. Accordingly, such all-plastic containers have not beencommercially accepted to any significant degree.

It has thus been one objective of the present invention to provide anall-plastic, flat container blank which can be converted to aheat-sealed, side-seamed, flattened tube form on conventional convertingequipment at relatively high commercial production speeds.

It has been a further objective of the invention to provide anall-plastic stock material which can be heat-sealed by the controlledapplication of direct heat to unconfined and thus unsupported surfacesthereof without undesired deformation, distortion or degradation of thematerial as a whole.

A further objective of the invention has been to provide a method formanufacturing and forming, at high production speeds, an all-plastic,side-seamed tube from a flattened all-plastic blank form, onconventional resin-coated paperboard carton converting equipment, andfrom which containers can be manufactured.

A further objective of this invention has been to provide a dynamicsystem for heat-sealing all-plastic container material at higherproduction speed than has been heretofore commercially available.

To these ends, the present invention broadly includes a stock materialof multiple layers wherein an outer layer of the multiple layer materialcan be rendered molten, in a dynamic system and when the material isunconfined, without distortion of the core area of material so that thecore provides structural undistorted, unshrivelled support for themolten outer layer. In a preferred embodiment, the invention includes anall-plastic, multiple-layer container blank comprising a "spine","backbone" or structural plastic core having a relatively high softeningpoint, and on each side of the core layer, an outer plastic layer,having a lower softening point. The material of the layers is selectedso that when the blanks are fed through conventional convertingequipment at predetermined speed, direct flame application to unconfinedportions of the blanks raises the temperature of the outer layer torender it suitably molten for heat-sealing, yet the core is notsignificantly thermally affected and provides structural rigidity andsupport, without distortion, for the softened and molten outer layers.

Accordingly, the core is preferably made, in one embodiment, from a highdensity polyethylene material while the outer layers are made from alower density polyethylene material. The thickness of the outer layersis selected so that suitable heat is retained to keep them sufficientlymolten until such time as one surface is pressed against another to forma heat-sealed side seam.

To insure integrity of the multi-layered composite, it is formed by aco-extruding process wherein the different core and outer layers arefirst joined in molten condition prior to simultaneous extrusion througha common die orifice. The inter-layer bond is thereby enhanced, in turncontributing to the integrity of the container's side seam and its topand bottom heat-sealed structures.

Thus, in one form of the invention, similar polymers for the core andouter layers are used, the core layer polymers comprising a higherdensity than the lower density outer layers. The softening pointtemperature differential between the two is maintained at a value whichpermits side-seaming by direct heat application in a dynamic system, asin conventional resin-coated paperboard converting equipment, tounconfined container portions but without deformation of the core.

The invention, including both the material and the dynamic method inwhich it is handled, is highly useful in a number of applications, suchas in forming folding boxes, containers, cartons, liquid-tightcontainers and other objects in which the utilization of all-plasticmaterials having sealing capabilities are desired.

In another aspect of the invention, where similar or compatible polymersfor core and outer layers are used, trim waste or scrap from theco-extruded stock material is added to the plastic supply for subsequentcore or outer layers in a ratio that will maintain the above mentionedtemperature differential which will permit heat-sealing of the materialin conventional converting systems. Also, complete containerrecyclability is provided by adjusting the softening points of portionsof the melt, comprising previous containers, and extruding it with otherportions having a different softening point. This enhances economy ofoperation and reduces waste, as compared to a resin-coated paperboardoperation where difficulties are encountered in economically removingthe resin and without damaging the paperboard substrate.

These and other features and advantages of the invention will becomereadily apparent from the following detailed description and drawings inwhich:

FIG. 1 is a view of a multiple-layer container blank in accordance withthe invention;

FIG. 2 is a diagrammatic end view showing the blank of FIG. 1 inpartially folded condition after heating;

FIG. 3 is an enlarged cross-sectional view of the circled area of theblanks of FIG. 2;

FIG. 4 is an enlarged cross-sectional view of a sealed blank inflattened tube form and in the area corresponding to that shown in FIG.3;

FIG. 5 is a diagrammatic elevational view of the converting apparatus;and

FIG. 6 is an end view taken along lines 6--6 of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The invention described herein is particularly useful for manufacturing,for example, containers of many various types. Such containers, forexample, may comprise folding boxes, square or rectangular containers orcartons, or simple cylindrical tubes having a bottom closure means andgenerally also a top closure means.

For example only, one particular form of container configuration withwhich the invention herein is highly useful is the gable-top liquid foodcarton described in the patents listed hereinabove. Accordingly, andmore fully to describe a preferred aspect of the present invention, thedisclosures of each of the above patents are incorporated by referenceas if they were set forth herein at length.

Of course, the multiple-layer composite described herein has many otheruses, as will be appreciated, and such as, for example, in the packagingof other foods and other materials where moisture, grease, petroleum,and the like are present and the packaging material must providebarriers to these elements.

Turning now to a detailed description of the invention, we havediscovered that it is now possible to dynamically side-seam anall-plastic material to form a tube, from which containers can be made,by heat-sealing the material via the direct predetermined, timedapplication of heat to unconfined relatively free-standing portions ofsaid material, without material distortion or shrivelling, and bythereafter pressing tacky portions of said material together, all athigh production speeds. Moreover, and for example, we have found thatall-plastic liquid-tight containers, according to our invention, can beheat-sealed at high speed on conventional resin-coated paperboard cartonconverting equipment.

The invention contemplates a multiple-layer or "composite" materialincluding a spine, backbone or core layer of thermoplastic polymer,which is essentially responsible for the structural rigidity of thepresent containers, and one or more outer layers which provide dynamicheat-sealability at high commercial production speeds, the relationshipof the physical characteristics of the core layer to those of the outerlayers being critical as defined herein. The particular composition ofeach respective layer is not in and of itself critical, but therelationship of certain physical characteristics of each layer to theother is critical.

In order to supply the various objectives of this invention, we haveprovided, in a preferred embodiment, a container material as above,where the physical characteristics of the core layer and the outerlayers exhibit a differential in softening point temperature. Thus, thecore and outer layer thermoplastic materials are selected so that thesoftening point of the core material is higher than that of the outerlayer material.

Further, the core layer material and outer layer material are selectedsuch that the outer container portions can be rendered sufficientlymolten, by the direct, timed application of heat, without distortion ofthe core layer which provides non-deforming structural support for thetacky outer layers. In this manner, not only can the impenetrability ofthe container be insured, but additionally, the all-plastic containerblank material can be simply and easily sealed, through dynamicapplication of heat to unconfined container portions without distortionor shrivelling of the container blank as a whole. Thus, the inventioncontemplates a dynamic system wherein a predetermined quantum of heat isapplied to a specified material of the above characteristics to achieveefficient heat-sealing at high speed.

Of course, the invention is not limited to any particular manner of heatapplication, it only being necessary that a sufficient amount of heat beapplied to soften and render the outer surfaces molten while leaving thecore substantially unaffected. Accordingly, reference is made herein toa "dynamic" application of heat which defines the application ofpredetermined heat to the material during a controlled time period. Aswill be seen, conveying the material through or past an open flame at apredetermined speed is one method of dynamically heating the material sothe described results are obtained.

In a preferred embodiment, a relatively high density polyethylene isselected as the core material while the same or similar polymer, but ofsignificantly lower density, is selected as the outer layer material.The differential in softening point temperature of the two materials issuch that the outer layer or layers can be dynamically heated tosufficiently molten condition for efficient heat-sealing withoutdistortion or shrivelling of the core as stated above.

Selection of the materials in this manner permits conversion of theall-plastic container provided herein on conventional convertingequipment now in use for heat-sealing resin-coated paperboard blanks toform tubes therefrom for later erection, filling and sealing in cartonform. Moreover, relatively high production speeds on the order of125,000 per hour are fully attainable.

As hereinafter described, the direct dynamic application of heat tounconfined container portions, in a conventional manner, rendered theouter surface molten, but did not distort or deform the core material ofthe present invention. The application of this same sealing technique tohomogeneous plastic containers failed, the plastic deforming andshrivelling into an unusable mass.

While the invention in its preferred embodiment contemplates a compositeincluding a core layer and an outer layer on each side thereof, itshould be appreciated that a composite including a core or support layerand a single outer layer on one side thereof has utility for someapplications as for example, in forming a heat-sealed pouch-typecontainer.

The core layer contemplated by the invention may be composed of anythermoplastic polymer material in accordance with the invention. It isordinarily preferred, however, that it be of high density, exhibitingfor example, a specific gravity of from about 0.95 to about 0.965, andmore preferably a density exhibiting a specific gravity of about 0.955to 0.960 or above, this density range insuring desirable moisture andvapor barrier characteristics for the container.

For structural rigidity, the thickness of the composite blank in aliquid food container application is approximately the same as that of aresin-coated paperboard container. Thus, the total thickness may be inthe approximate range of from 15 mils to about 25 mils, although thisthickness may vary outside this range depending on specific application.Thickness of the core layer may thus be in the approximate range of fromabout 13.5 mils to about 23.5 mils, depending on application and, ofcourse, on the physical characteristics of the particular materialselected. This insures a desirable rigidity and strength in the productcontainer. Most preferably, for use in a liquid food container, corelayer thickness is approximately from about 15 mils to about 17.5 mils.

Suitable examples of the preferred core layer material are thethermoplastic resins such as polyolefin, polyvinyl chloride,polystyrene, polyvinyl acetate and copolymers thereof and the like. Mostpreferred are polyethylene, polypropylene, and copolymers thereof. Theyare well-known in the art as exhibiting optimal packagingcharacteristics.

The outer layers which are fused to either side of the core layer areprimarily responsible for the heat-seal characteristics of the containermaterial. In contrast to the polymer of the core layer, which remainsessentially inert during the production of the present containers, theouter layers must be capable of softening to a molten conditionsufficient for fusing where a seal is desired; and at a temperature lessthan that which would tend to distort or shrivel the core when heatedwhile unconfined.

Of course, at ambient temperatures, such as those ordinarily encounteredincident to use, the outer plies remain hard and fluid-impenetrable.Thus, they further contribute to the desirability of the container as awhole.

The outer layers may generally be composed of any of the polymericmaterials already described with respect to the core ply material and inaccord with the disclosure herein. Preferably, the outer layers shouldbe susceptible to printing and thus in one embodiment comprisepolyethylene which insures use of direct printing without the need forthe application of an independent label.

In a preferred embodiment, density of the outer layers is lower thanthat of the core. Generally, a low density thermoplastic material, suchas polyethylene, exhibiting a specific gravity in the approximate rangeof from about 0.918 to about 0.930 is chosen. Most preferable are suchmaterials exhibiting a specific gravity of from about 0.922 to about0.925. A thermoplastic material exhibiting a specific gravity of about0.9245 has been found particularly useful.

The thicknesses of the outer layers are, like those of the core layer,susceptible to wide variation depending on application. In a usefulapplication of the invention for liquid food containers, the outerlayers may have a thickness in the approximate range of from about 5% toabout 15% of the thickness of the core layer, and preferably about 10%of the core thickness. Of course, the outer layer's thickness may beoutside this range depending on application and on production apparatusfor producing the layers and for treating the composite material. Forexample, very thin outer layers (less than 5% of core thickness) couldbe utilized if they could be heated and then sealingly joined veryquickly.

Preferably, however, in liquid food containers, these outer layers areof a thickness of from about 0.50 mils to about 2.5 mils and mostpreferably from about 0.75 mils to about 2.0 mils. Such a thickness,where the core layer is about 15 to 17.5 mils, provides properheat-sealability of the outer layers, while avoiding excess thickness inthese layers, and permitting the core to provide the necessarystructural rigidity.

Selection of the outer layer's thickness is also important to efficientheat-sealing of the material. For example, in conventional convertingequipment heat is applied at one station while the compression nip, forpressing heated molten portions together, is spaced at anotherdownstream station. The heated plastic of the container materials tendsto retain heat, and thus desired fluidity, in relationship to the volumeof heated plastic. Thus, the plastic thickness must be such that itremains sufficiently molten, after heating, until it is joined with acorresponding molten surface. Too little plastic thus may not retainsufficient heat for effective sealing.

While various known techniques may be used to form the multiple-layercomposite of core and outer layers described herein, it is preferable toproduce them by the known technique of co-extrusion in order to preventlayer separation. Lamination techniques, where the layers are joinedsubsequent to extrusion may not provide adequate layer adhesion orfusion.

In a preferred technique, the materials of the core and outer layers aresimultaneously extruded through a common die orifice, having been joinedin molten form prior to extrusion. This technique provides improvedlayer integrity, positively fusing them together, and enhances theintegrity of the side seam seal and the seal at the container's top andbottom structure.

In addition to the thermoplastic polymers of which the present layersare composed, various additional elements may be included asconstituents of one or more of the layers in order further to improvethe present containers. Thus, for example, one or more layers may beprovided with a pigment such as titanium dioxide. Such a pigment, whichis normally dispersed throughout one or more layers, enhances theappearance of the present containers.

In addition, however, pigments may be utilized in order to provideprotection for the eventual container contents. Thus, for example, wherelight may adversely affect the eventual contents of a container,appropriate dispersion of pigments in, or on, one or more layers may beutilized to render the container opaque. This increases the storagestability for the contained liquid.

After manufacture of the co-extruded stock material, which is normallycollected in rolls, the manufacture of the present containers proceeds.The multiple-layer material is unrolled and scored and cut intoappropriate configuration for forming the container. In this connection,we have found it advantageous to cut and score the plastic material inaccordance with U.S. Pat. Nos. 3,594,464 and 3,768,950, to avoiddistortion on the side wall panels of the container blank. Preferably,the material is cut into blanks and scored in the score line patternshown in FIG. 1.

An exemplary blank 10 includes side wall panels 11-14, a glue flap 15,extending from side panel 11, and a side portion or edge 16 (oppositeside) of panel 14 to which the glue flap will be heat-sealed. The blankfurther comprises a top forming structure 17, formed by the cut andscored top panels extending from the side panels as shown, and a bottomforming structure 18 formed by the cut and scored bottom panelsextending from the side panels as shown. In addition to other scorelines, the blank includes score lines 25 and 26 about which the blank isfolded, after heating, to form a flattened tube.

As shown in FIGS. 3 and 4, the blank is a multiple-layer compositepreferably of three layers including respective outer layers 20 and 21and a core layer 22. As stated above, the materials are preferablythermoplastic polymers, the core being of higher density than the outerlayer polymers, and having a softening point temperature greater thanthat of the outer layer polymer so that the outer layers can be heated,when unconfined, to a softened molten condition suitable forheat-sealing without distortion or shrivelling of the core.

After cutting and scoring, the blanks are converted into flattened tubeform, on conventional resin-coated paperboard converting equipment, byheat-sealing a side seam therein so that significantly, no adhesives,staples or other agents are required to retain the seam. Afterconverting, the containers are shipped to a dairy, for example, or otheruser where they are erected, filled and sealed.

In the converting state, FIGS. 5 and 6 depict a conventional heat-seal,side-seamer apparatus wherein a plastic multiple-layer blank 10, inlaid-out flat form, is conveyed past a heating station 35. A conveyorcomprising, for example, belts 36a, 36b, 36c and 36d, carries successiveblanks 10, in the machine direction indicated by Arrow A, such that theedge portion 16 and glue flap 15 of each are in an unconfined condition.The conveyor is driven at a line speed of about 1800 feet per minute.

At the heating station, a pair of burner lines comprising elongatedburners 37 and 38 are aligned adjacent the conveyor to direct a flame Fat approximately 1700° F. against an elongated area 30 extending alongthe glue flap 15 from carton top structure to carton bottom structureand an elongated area 31 extending along the side wall edge portion 16from carton top structure to carton bottom structure. Each line ofburners is, in total, approximately 48 inches long and has numerous gasoutlets sufficient to provide a nearly continuous flame along theburner's length. Each burner line may comprise two or more burners asshown, for example, at 37a and 37b. The burners are oriented to heat theappropriate sides of the blank as shown.

At the heating station, water cooled bars 39-42 provide guides for theblank as it passes the burner. In this respect, the blanks may contactthe bars, opposite the burners, but the bars only guide and do notprovide confining structural support for the blank as would keep it fromshrivelling. Thus, in the context of this application, the term"unconfined" is defined to mean without confinement or engagement onboth sides of the sealing area, as in one conventional heat-sealingoperation wherein heated members engage and confine, in supportingrelationship, the material to be sealed.

Application of the heat to the moving outer plastic layers 20 and 21 inthe respective areas 30 and 31 on the glue flap 15 and edge portion 16is sufficient to render them sufficiently molten for efficientheat-sealing, yet the core is not softened to such an extent that itwould distort or shrivel. Thus, although the outer layers are heated tobecome sufficiently molten throughout a flat sealing area, the corelayer is so thermally insensitive to the heat for the duration of timetherein that the core provides undistorted structural support for themolten outer layers, even though the edges of the blank are notstructurally confined.

After application of heat, the blank is folded at a folding station 44about score lines 25 and 26 and the glue flap 15 and edge portion 16 aresealingly joined at a downstream nip 45. The nip comprises rollers 46and 47, and is spaced about 6 or 7 feet from the end of the burner line.

Configuration of the partially folded blank is shown in cross-section inFIG. 2, and in more detail at the side seam area of FIG. 3. In FIG. 3,the outer layers 20 and 21 have been softened and rendered molten in theelongated areas designated at 31 and 30 respectively. As shown, the corelayers provide structural support for the outer layers, remainingundistorted and unshrivelled.

FIG. 4 discloses a detailed cross-section of the side-seamed flattenedtube in the same area depicted in FIG. 3, but with the seal completed.As shown, the molten outer layers, in the areas 30 and 31, have fused toform a liquid-tight side seam between the glue flap 15 and edge portion16 of side wall panel 14.

The tubes thus formed, and after sealing, are then typically transportedto a user such as a packaging facility or a dairy, for example, wherethey are erected, bottom sealed, filled, and top sealed. Both bottom andtop sealing can include the dynamic application of heat to appropriateunconfined surfaces of sealing panels to render the outer layer orlayers sufficiently molten to provide a liquid-tight seal, all withoutdistorting or shrivelling of the core layer.

Although apart from the softening point differentials specified above itis not always necessary that the thermoplastic polymer of the core or ofeither of the outer layers be selected having regard to the types ofpolymer of any other layer, it is often desirable to pick at leasthighly compatible polymers if the same polymer (differing in densityonly) is not used. Substantial advantages can be obtained where, forexample, the present containers are to be recycled.

Both outer layers of the present composite are therefore preferablycomposed of the same polymer. Further, in certain recycling techniques,it is still more advantageous if all layers are composed of the sametype of polymer.

In this regard, and in a preferred recycling operation, any scrap ortrim waste produced incident to manufacture of stock material or blankscan be readily recycled by adding it to the plastic supply for eithercore or outer layer in such a ratio as will not reduce the minimumsoftening point differential desired between the core layer and theouter layers. Where such scrap or waste is to be recycled, therespective layer materials can be originally selected to permit theaddition of scrap or waste in the ratio, for example, of one part scrapor waste, to nine parts of original core plastic without adverselyaffecting the minimum required parameter between the softening points ofthe two layer materials.

A further significant advantage of this invention lies in the completerecyclability of the present containers. Obviously, the ability toreclaim all constituents of a container material presents an economicadvantage. Thus, the present container material is susceptible tosimplified and inexpensive recycling techniques which further increaseits desirability.

More particularly, while recycling may be accomplished by a number ofreadily apparent techniques, we provide a particularly effective processwherein the entire thermoplastic material can be utilized for theproduction of further containers or any of the myriad products of whichsuch a material is commonly composed.

While a molten mass of the composite material will exhibit a softeningpoint intermediate that of the initial core and outer layers, thematerial is still capable of recycling into new container material.Thus, the molten admixture may be utilized for formation of either acore layer or one or more outer layers of a new composite material. Allthat is required is that the thermoplastic polymer of other layer orlayers be selected having regard to the already discussed softeningpoint differentials required for such layers.

A further technique for reducing the amounts of additional materialsnecessary for the formation of multiple-layer composites includesseparating the molten admixture, formed from the initial containercomposite material, into at least two like portions. One or more of suchportions may then be adjusted, through addition of an agent which willalter its softening point. For example, a polymer of very high softeningpoint may be added to one of the two portions so as to increase itssoftening point, with respect to the other portion, by at least therequired number of degrees to be useful, in accordance with thisinvention, as a core layer for a lower softening point predeterminedouter layer material. Thereafter, the portion with higher softeningpoint may be formed into a core layer while the remaining portion orportions are applied thereto as outer layers by co-extrusion.

Conversely, an agent such as a relatively low softening point polymercan be added to one or more of the portions. In this instance, it is theportion not containing the additive which is utilized for forming thecore layer of the recycle composite.

The following specific examples will serve further to illustrate thepractice and advantages of the present invention.

EXAMPLES

A composite multilayer sheet was prepared by co-extrusion, each sheetcomprising a core layer, of relatively high density polyethylene, andouter layers, of relatively lower density polyethylene, on each side ofthe core layer. Various sheets were formed according to the followingspecific examples.

    ______________________________________                                                         Approx.  Approx.                                                              Density  Soften-                                                       Material                                                                             gms./ml. ing Point                                                                              Thickness                                  ______________________________________                                             Core       PE 9420  .955   129° C.                                                                       15.0 mils                              #1   Outer Layers                                                                             PE 4524  .9245  110° C.                                                                        1.0 mil                                    Core       PE 9420  .955   129° C.                                                                       15.5 mils                              #2   Outer Layers                                                                             PE 5554H .9245  110° C.                                                                        .75 mil                                    Core       PE 9420  .955   129° C.                                                                       15.0 mils                              #3   Outer Layers                                                                             PE 5554H .9245  110° C.                                                                        1.0 mil                                    Core       PE 9420  .955   129° C.                                                                       17.5 mils                              #4   Outer Layers                                                                             PE 5554H .9245  110° C.                                                                        .75 mil                               ______________________________________                                    

Softening points of the specific polymer materials set out in the abovesamples are only approximate. The temperatures are exemplary only, andhave been included herein on the basis of information provided by GulfOil Chemicals Company, U.S. Operations, Plastics Division, OrangeTechnical Laboratory. The softening points were determined according toStandard Tests Nos. 201 and 202 of the same Laboratory.

On the basis of this information and these specific tests, thetemperature differential between the high density core and the lowerdensity surface is about 20° C. As stated herein, the exact softeningpoints are not critical, but the softening points exhibited by thepolymers of type and density as noted are sufficient to permit renderingthe outer layers molten, on conventional thermoplastic coated paperboardcarton converting apparatus as will now be set out, while the coreremains significantly unaffected thermally so as to provide solestructural support for the outer layers.

A tube was formed from these sheets by side-seaming on conventionalresin-coated paperboard converting apparatus as described herein, theunconfined moving container portions being heated so the outer layersbecame sufficiently molten to provide an efficient heat-seal. Line speedwas approximately 1800 feet per minute and burner flame temperatureabout 1700° F.

Despite this dynamic heating step, the core layers did not exhibit anysign of distortion or shrivelling and provided structural support forthe molten outer layers. Moreover, the seal formed by the fused outerplies was continuous and liquid-tight.

Strengths of the sealed side seam were tested, as was the side seam of aconventional resin-coated paperboard control container sealed onconventional converting equipment. Both seal strength (tensile pullalong seal) and heat-seal strength (seal shear) were found to becomparable, and in some examples, superior to the strength exhibited bythe control carton.

Accordingly, and in one form, the invention provides an all-plasticcontainer material capable of being side-seamed into a flattened tubeform at high speeds on conventional resin-coated paperboard carton blankconverting equipment, thereby rendering the all-plastic cartoncommercially practicable for liquid-tight as well as other containers.

These and other advantages, modifications and equivalents will becomereadily apparent from this disclosure, to those of ordinary skill in theart, without departing from the scope of this invention, and applicantsintend to be bound only by the appended claims.

We claim:
 1. A method of making a multiple-layer composite blank productin flattened tube form from which containers can be made, said blankcomprising a composite sheet having a core layer of high densitythermoplastic polymeric material having a specific gravity of about0.950 to about 0.965, a thickness of 15 mils to about 17.5 mils, and afirst pre-determined softening point, and at least one outer layer oflower density thermoplastic polymeric material on a side of said corelayer, said outer layer having a specific gravity of about 0.918 toabout 0.930, a thickness of about 0.75 mils to about 2.0 mils, and asecond pre-determined softening point lower than said firstpre-determined softening point such that said outer layer, at unconfinededge portions of said blank can be dynamically heated to sufficientmolten condition for heat sealing while said core layer is notsignificantly thermally affected, and provides structural support forsaid outer layer without distortion or shrivelling, said methodcomprising the steps ofco-extruding said layers to form a compositesheet having a core layer and an outer layer; cutting and scoring saidsheet to form said composite container blank; passing unconfinedportions of said composite container blank through an open flame, saidflame impinging on said unconfined portions; thereby heating said outerlayer by said flame to a molten condition for heat-sealing, while saidcore layer remains thermally insensitive to said heat and structurallysupports said tacky outer layer in an undistorted condition; and,joining tacky portions of said composite container blank to form aside-seamed tube.
 2. A method, as in claim 1 wherein, said core layerprovides the sole structural support for said unconfined molten outerlayers.
 3. A method of making a multiple-layer composite blank productin flattened tube form from which containers can be made, said blankcomprising a core layer of high density thermoplastic polymeric materialhaving a specific gravity of about 0.950 to about 0.965, a thickness of15 mils to about 17.5 mils, and a first pre-determined softening point,and an outer layer of lower density thermoplastic polymeric material oneach side of said core layer, said outer layers having a specificgravity of about 0.918 to about 0.930, a thickness of about 0.75 mils toabout 2.0 mils, and a second pre-determined softening point lower thansaid first pre-determined softening point such that said outer layers atunconfined edge portions of said blank, can be dynamically heated tosufficient molten condition for heat-sealing, while said core layer isnot significantly thermally affected, and provides structural supportfor said outer layer without distortion or shrivelling, said methodcomprising the steps of:co-extruding said layers to form a compositesheet having a core layer and an outer layer on each side thereof;cutting and scoring said sheet to form a composite container blank;passing unconfined portions of said composite container blank through anopen flame, said flame impinging on said unconfined portions; therebyheating said outer layers by said flame to a molten condition forheat-sealing, while said core layer remains thermally insensitive tosaid heat and structurally supports said tacky outer layers in anundistorted condition; and, joining tacky portions of said compositecontainer blank to form a side-seamed tube.
 4. A method, as in claim 3wherein, said core layer provides the sole structural support for saidunconfined molten outer layers.